1. Field of the Invention
The present invention relates to hydraulic systems that control fluid flow to a hydraulic actuator which moves a mechanical component on a machine, and in particular to recovering energy from the hydraulic actuator and utilizing the recovered energy subsequently to power the hydraulic actuator.
2. Description of the Related Art
Construction and agricultural equipment employ hydraulic systems to operate different mechanical elements. For example, an excavator is a common construction machine that has boom pivotally coupled at one end to a tractor and having a bucket at the other end for scooping dirt and other material. A cylinder assembly is used to raise and lower the boom and includes a cylinder with a piston therein which defines two chambers in the cylinder. A rod connected to the piston is typically attached to the boom and the cylinder is attached to the body of the excavator. The boom is raised and lowered by extending and retracting the rod out of and into the cylinder.
Other machines use different types of hydraulic actuators to produce motion of a mechanical element. The term “hydraulic actuator”, as used herein, generically refers to any device, such as a cylinder-piston arrangement or a rotational motor for example, that converts hydraulic fluid flow into mechanical motion.
During powered extension and retraction of the cylinder assembly, pressurized fluid from a pump is usually applied by a valve assembly to one cylinder chamber and all the fluid exhausting from the other cylinder chamber flows through the valve assembly into a return conduit that leads to the system tank. Under some conditions, an external load or other force acting on the machine enables extension or retraction of the cylinder assembly without significant fluid pressure from the pump. This is often referred to as an overrunning load. In an excavator for example, when the bucket is filled with heavy material, the boom can be lowered by the force of gravity alone. That external force drives fluid out of one chamber of the boom's hydraulic cylinder through the valve assembly and into the tank. At the same time, an amount of fluid is drawn from the pump through the valve assembly into the other cylinder chamber which is expanding, however because that incoming fluid is not driving the piston, it does not have to be maintained at a significant pressure for this boom motion to occur. In this situation, the fluid is exhausted from the cylinder under relatively high pressure, thereby containing energy that normally is lost when the pressure is metered through the valve assembly.
To optimize efficiency and economical operation of the machine, it is desirable to recover the energy of that exhausting fluid, instead of dissipating it in the valve assembly. Some prior hydraulic systems sent that exhausting fluid to an accumulator, where it was stored under pressure for later use in powering the machine. However, a challenge to efficient energy recovery and reuse is that the stored hydraulic fluid has to be at the proper pressure and volume to power an actuator. The relationship between the pressure and volume of the exhausting fluid and those parameters of the accumulator varies instantaneously and determines whether that fluid can be stored. For example, if the external force acting on the cylinder assembly is insufficient to pressurized the exhausting fluid above the level of pressure in the accumulator, then that fluid cannot be stored.
At another time when use of the fluid in the accumulator is desired, the instantaneous relationship between the pressure and volume of the accumulator and that required of the fluid to power the hydraulic actuator determines whether the accumulator fluid can be used. For example, if the load on the hydraulic actuator requires a greater pressure than the accumulator pressure, then the recovered fluid cannot be employed. Also if the hydraulic actuator needs to move so far as to require a greater volume of fluid than is stored in the accumulator, effective operation may be difficult to achieve. Another limiting factor is that as the hydraulic actuator consumes fluid from the accumulator, the accumulator pressure decreases reducing the ability of the remaining fluid to power the actuator.
Therefore, a need exists to provide an effective techniques for recovering and reusing energy in a hydraulic system.